In recent years there has been a growing acknowledgement that many breast cancers that are diagnosed and treated would never have posed a threat to a woman's health or well-being during her lifetime. Such cancers are indolent or slow-growing, but when detected by breast cancer screening, cannot be distinguished from cancers that could be life-threatening. Such cancers are often referred to as over-diagnosed. Among the established harms that over-diagnosed cases may experience are emotional consequences such as depression and anxiety as well as the adverse effects of the therapy itself, including lymphedema, bone loss and fracture, cardiovascular disease, and second primary tumors. However, despite general consensus that over-diagnosis is unacceptably high and that adverse efforts in such cases must be reduced, relatively few efforts have been devoted to identification of characteristics of breast cancers that might be considered over-diagnosed. Thus, there is a critical need to identify clinical, pathologic, and biological factors f cancers that have similar outcomes regardless of treatment, or with only minimal (surgical) treatment. We propose to examine the natural history of untreated and minimally treated breast cancer in a population of over 2000 in situ and 2000 invasive breast cancer cases in older women who received no treatment, as well as over 25000 and 80,000 women, respectively, who received only surgical treatment, and compare their outcomes to similar but more extensively treated women followed for 10 years. In this study, we will provide additional supporting evidence for lesions that can be termed IDLE - indolent lesions of epithelial origin. The analysis will also produce data regarding the time frame available for observation (active surveillance) in untreated cases before treatment may be needed, supporting the possible development of observational registries. Data regarding the characteristics of lesions that when untreated do not lead to excess mortality within 10 years may also advance the aim of raising the threshold for biopsy, possibly allowing a woman to be enrolled in active surveillance instead. The study should contribute to reduction of harm in up to 25% or more of diagnosed breast cancer cases, while maintaining beneficial outcomes and preventing breast cancer death.